What typically contributes to the density difference between saltwater and freshwater?

The density difference between saltwater and freshwater is primarily influenced by the mineral content, specifically the concentration of salts dissolved in the water. Saltwater contains a significant amount of dissolved salts, mainly sodium chloride, which increases its mass per unit volume compared to freshwater that has very low levels of dissolved salts. This higher concentration of minerals in saltwater makes it denser, allowing it to sink beneath less dense freshwater when the two meet.

Other factors like temperature variations can also affect density, as warmer water is generally less dense than cooler water. However, in the context of comparing saltwater and freshwater, the primary factor that defines the density difference is indeed the mineral content. Oxygen levels and hydraulic pressure have less direct influence on the basic density differences observed between these two types of water.